Synthesis Of Silyl Fe,Co And Ni Hydrides And Their Applications In Catalytic Reactions

Transition metal hydrides have been playing an important role in organometallic chemistry.They are widely used in organic synthesis,pharmaceutical industry,energy transfer and other fields because of their excellent catalytic properties.In particular,cheap metal hydrides with wide sources and low cost have attracted the attention of scientists.In recent years,the application of metal hydride in nitrogen reduction has attracted more and more attention of chemical workers.Nitrogen in the air is the most ideal nitrogen source.However,due to the high bond energy of nitrogen-nitrogen triple bond,it is difficult to use this ideal nitrogen source efficiently.At present,the common nitrogen fixation methods are Haber Bosch industrial process.However,Haber industrial process requires extremely harsh conditions and high energy consumption.In recent years,the reduction of nitrogen to ammonia,silyl amine and other nitrogen-containing compounds under mild conditions using transition metal nitrogen complexes as catalysts has become a hot topic in organometallic chemistry.Inspired by the structure of nitrogenase,the nitrogen complexes of molybdenum and iron have been studied extensively and deeply.In particular,iron complexes,which are abundant in reserves,cheap and easy to obtain,are favored by chemists.In transition metal nitrogen complexes,the formation of metal-nitrogen backbonds is very important for the activation of nitrogen.The properties of ligands directly affect the electrical properties of metal centers and the formation of metal-nitrogen backbonds.Therefore,ligands play an important role in the formation and properties of nitrogen complexes and hydrides.The[PSi]type ligands have excellent performance,excellent a donating ability and π backbond accepting ability,which are easy to stabilize metals,and the trans-effect of silicon is also conducive to enhance the activity of metal complexes.Therefore,silyl complexes have great potential in the field of catalysis,which attracts more and more attention of scientists.In this paper,a series of bidentate and pincer type silyl-iron,cobalt,and nickel hydrides and nitrogen complexes were synthesized by activation of Si-H bond,and their catalytic properties were explored,especially their application in nitrogen reduction.It can be divided into the following four aspects:1.The synthesis and properties of novel silyl iron dinitrogen hydrides.A variety of novel phosphorus-anchored silyl ligands were designed and synthesized by changing the substituents on the phosphorus atom and silicon atom.The new silyl iron hydrides and nitrogen complexes were constructed by reacting with the electron-rich iron precursor supported by trimethylphosphine.The results show that the formation of nitrogen complexes is favorable when electron-donating groups are attached to the ligands and the steric hindrance is moderate.The silyl iron nitrogen hydrides can react with hydrogen at room temperature to form polyhydric complexes,which indicates that these nitrogen complexes can activate hydrogen at room temperature.In addition,we focused on the study of nitrogen silylation catalyzed by iron nitrogen hydride.By optimizing the reaction conditions,the reduction of nitrogen to silylamine catalyzed by iron nitrogen hydride was realized under mild conditions,and good catalytic effect was achieved.2.Lewis acid promoted dehydration of amides to nitriles catalyzed by[PSiP]-pincer iron hydrides.The dehydration of primary amides to their corresponding nitriles using four[PSiP]-pincer hydrido iron complexes 16-19 as catalysts in the presence of{EtO)3SiH as dehydrating reagent was explored in the good to excellent yields.It was proved for the first time that Lewis acid could significantly promote this catalytic system under milder reaction conditions than other Lewis acid promoted system,such as shorter reaction time or lower reaction temperature.This is also the first example that dehydration of primary amides to nitriles was catalyzed by silyl hydrido iron complexes bearing[PSiP]-pincer ligands with Lewis acid as additive.This catalytic system has good tolerance for many substituents.Among the four iron hydrides 16 is the best catalyst.The effects of substituents of the[PSiP]-pincer ligands on the catalytic activity of the iron hydrides were discussed.A catalytic reaction mechanism was proposed.3.Pyridine N-oxide promoted hydrosilylation of carbonyl compounds catalyzed by[PSiPI-pincer iron hydridesFive[PSiP]-pincer iron hydrides 10,16-19 were used as catalysts to study the effects of pyridine N-oxide and the electronic properties of[PSiP]-ligands on the catalytic hydrosilylation of carbonyl compounds.It was proved for the first time that this catalytic process could be promoted with pyridine N-oxide as the initiator at 30℃ because the addition of pyridine N-oxide is beneficial for the formation of an unsaturated hydrido iron complex,which is the key intermediate in the catalytic mechanism.Complex 18 as the best catalyst shows excellent catalytic performance.4.The synthesis of novel silyl cobalt and nickel hydrides.Three cobalt hydrides and two nickel hydrides were synthesized via the activation of Si-H bond through the reaction of electron-rich cobalt or nickel complexes with[PSi]-chelate ligands.The new complexes were fully characterized.